900 MHz or 2.4 GHz what is better - Page 3 - RC Groups
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Jan 16, 2008, 07:44 PM
Dick Corby
altacom's Avatar
Quote:
Originally Posted by Julez
I agree on this.
We all know that neither 72MHz nor 2.4GHz work well inside CF. Why should any frequency in between work better?
From what I read, the longer antenna for 900 MHZ will allow it to be hanging out of the Fuselage, similar to 72 MHZ but much shorter and less ovtrusive.

And I do wish Xjet and Schubub would go to the dedicated thread and quit clogging every thread started with their constant bickering.
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Jan 17, 2008, 06:06 AM
Proud to eat Kraut ;-)
Julez's Avatar
Quote:
From what I read, the longer antenna for 900 MHZ will allow it to be hanging out of the Fuselage, similar to 72 MHZ but much shorter and less ovtrusive.
Well, but that is just what Xjet said:

Quote:
900MHz won't travel through carbon significantly more easily than 2.4GHz will. The advantage of 900MHz in a carbon plane is that the antenna is long enough that you can route it outside the fuselage.
But you replied:

Quote:
Wrong again. Try some tests with a carbon fiber fuse at 2.4 and 900. Weave is important too.
Now I ask for the reason, why 900MHz is supposed to penetrate CF better than the frequencies around it, ans you answer exactly what Xjet said, despite claiming he was wrong.

Something does not make sense her
Jan 17, 2008, 08:00 PM
Registered User
skubacb's Avatar
From the testing I have heard about and read 900mhz goes through CF easier than 2.4ghz. Mind you this is not my testing. Longer wave lengths seem to penetrate CF easier.

Why? Maybe for the very same reason 2.4ghz is at the resonant wave length of water and therefore works great for a microwave. It excites the water molecules.
(I'm wrong, no idea where my head was at. 2.4ghz transfer its energy to water)

900 mhz does not have the same affect on water. Why does the weave affect penetration. Take a close look at a radar reflector. They are not solid but are a metal "weave" at the correct spacing to reflect the radar signal. I do not know the interaction with all types of materials on all frequencies. But the wave length does affect the interaction.

The 900mhz system I saw at AMA (if I remember correctly) did not have some long antenna hanging behind it. Jim did say something about getting better penetration with a particular CF sailplane. Of course we have to wait for the production model to know what the configuration will actually be.
Last edited by skubacb; Jan 19, 2008 at 10:32 AM.
Jan 17, 2008, 09:39 PM
Registered User
Radar antennas aren't solid to save weight and also lessen effect of wind. However, CF as we use it is so tightly held together that for all radio, and even light waves it's more or less solid. So I don't know if there would be huge difference between 900 and 2.4 GHz.

The weave might have an effect depending on the polarization of the EM waves. Those radar reflectors most of the time are made from vertical rods. Turn them sideways and it wouldn't work very well as a reflector. However, in a RC plane getting the weave and radio polarization together to get proper penetration I don't think is easily possible. I guess people can try.
Jan 17, 2008, 10:04 PM
Registered User
Quote:
Originally Posted by skubacb
...
Why? Maybe for the very same reason 2.4ghz is at the resonant wave length of water and therefore works great for a microwave. It excites the water molecules. ....
I don't know where you got that information, but it is not exactly correct. Microwave ovens use ~2.45 gHz because it is absorbed weakly enough in liquid water (not free water molecules) that the waves maintain good strength even deep inside a typical piece of food. Higher frequencies would penetrate less well and cook less evenly. Lower frequencies would penetrate better, but would be absorbed so weakly that they wouldn't cook well. If you used the resonant frequency of water, nothing would ever cook below the surface.
Jan 17, 2008, 10:13 PM
Registered User
What is that resonant H2O frequency?
Thanks.
Jan 17, 2008, 10:21 PM
Registered User
22.235 gHz is the lowest resonant frequency of a water molecule. Or about 10 times what a microwave oven or XPS operates on.
Jan 17, 2008, 10:26 PM
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tychoc's Avatar
Quote:
Originally Posted by skubacb
The 900mhz system I saw at AMA (if I remember correctly) did not have some long antenna hanging behind it. Jim did say something about getting better penetration with a particular CF sailplane. Of course we have to wait for the production model to know what the configuration will actually be.
The length of the antenna is easy to calculate. The approximate formula for the wavelength is: lambda=c/f

So, for 900Mhz you get a wavelength of 3e+8/900e+6 = 0.33 meters. So a typical 1/4 dipole (with or without magical sperical radiators ) becomes
8.33 cm or 3.28 inches. Thus, the length of the antenna should be around 3 inches.

-tychoc
Jan 18, 2008, 05:56 AM
Registered User
skubacb's Avatar
(No idea where my head was at on the resonant freq. I was wrong)

Ok, the microwave thing is actually more complicated if we want to get into it, "The free water molecule possesses a large permanent dipole moment, which responds easily to the changing electric field vector at frequencies below the microwave region of the electromagnetic spectrum. At microwave frequencies, however, these polar groups cannot respond as easily to the field, and an absorption of energy results, peaking at slightly above 1011 Hertz. Any interference with dipole reorientation will alter the absorption with respect to that of free water."

I don't care. A wave length of 2.4ghz is absorbed more easily in water than 900mhz.

On the orientation of the poles. It depends on the polarization of the signal you want to reflect. Yes, the radar reflector has holes for wind, we know that. That is not the point. The point is that if the holes are too big it will not reflect the signal. It can be almost transparent to a specific signal. The weave/grid is important to the reflection of the signal.

Did anyone notice that some people have tested (not my testing) and that was their observation on the effect of CF?
Last edited by skubacb; Jan 19, 2008 at 10:33 AM.
Jan 18, 2008, 07:04 AM
Registered User
Quote:
Originally Posted by skubacb
Ok, the microwave thing is actually more complicated if we want to get into it, The free water molecule possesses a large permanent dipole moment, which responds easily to the changing electric field vector at frequencies below the microwave region of the electromagnetic spectrum. At microwave frequencies, however, these polar groups cannot respond as easily to the field, and an absorption of energy results, peaking at slightly above 1011 Hertz. Any interference with dipole reorientation will alter the absorption with respect to that of free water."

I don't care. A wave length of 2.4ghz is absorbed more easily in water than 900mhz.

On the orientation of the poles. It depends on the polarization of the signal you want to reflect. Yes, the radar reflector has holes for wind, we know that. That is not the point. The point is that if the holes are too big it will not reflect the signal. It can be almost transparent to a specific signal. The weave/grid is important to the reflection of the signal.
Please stop copying/pasting stuff from the Internet and write about things you understand better! With your own words this time!


Quote:
Originally Posted by skubacb
Did anyone notice that some people have tested (not my testing) and that was their observation on the effect of CF?
Did you notice that their range was still desperately short? And they had to come up with other tricks to improve it?
Jan 18, 2008, 12:55 PM
Proud to eat Kraut ;-)
Julez's Avatar
Quote:
Longer wave lengths seem to penetrate CF easier.
Why does 72MHz not work inside CF fuselages, then?
Jan 18, 2008, 06:01 PM
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tadawson's Avatar
Interestingly enough, when I was with Motorola years ago doing RF transmission system design for Cellular, and when Cellular telephones were just being released into the world (and on 900MHz), the reason for the choice of 900 at the time (well, one of the reasons . . . ) was that 900 had better building penetration than other frequencies - higher or lower. The lower stuff would not reflect into the structures, and the higher stuff would get eaten . . . 900 would just reflect into hallways, etc. and cover inside a structure quite well. Not sure if this has any bearing on this topic at all, but perhaps 900 could get *into* a CF structure via and opening, and propagate within it better than the others . . . it would be consistent with what we saw, albeit in much larger scale environments . . .

FWIW . . . .

- Tim
Jan 19, 2008, 12:56 AM
Registered User
skubacb's Avatar
HarrisM, I understand it and quoted it. My point is I care about the real world effect of the freqs we are using.
-------
Something such as what TADawson may explain some things or not. Theory is one thing and real world effects is another.

If Jim states that the 900mhz system will have a range somewhere about what the 2.4ghz system has then why not wait and see rather than posting theory or conjecture about the system or stating it will not do that. When it is out or he releases more details we will see.

Lets look at how the system actually should work (edited)
Last edited by skubacb; Jan 19, 2008 at 10:35 AM.
Jan 19, 2008, 01:38 AM
Registered User
Quote:
Originally Posted by Julez
Why does 72MHz not work inside CF fuselages, then?
Cause CF is a conductive material and as I stated before it's more or less solid to RF waves. Some just don't listen then take a bat to a dead horse.
Jan 19, 2008, 01:40 AM
Registered User
Quote:
Originally Posted by tadawson
Interestingly enough, when I was with Motorola years ago doing RF transmission system design for Cellular, and when Cellular telephones were just being released into the world (and on 900MHz), the reason for the choice of 900 at the time (well, one of the reasons . . . ) was that 900 had better building penetration than other frequencies - higher or lower. The lower stuff would not reflect into the structures, and the higher stuff would get eaten . . . 900 would just reflect into hallways, etc. and cover inside a structure quite well. Not sure if this has any bearing on this topic at all, but perhaps 900 could get *into* a CF structure via and opening, and propagate within it better than the others . . . it would be consistent with what we saw, albeit in much larger scale environments . . .

FWIW . . . .

- Tim
We should be bidding, as RC flyers, for some of that UHF bandwidth being freed up with digital TV. I hear cell companies are very very interested since that gets into building even better.


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